This invention relates to bending heat softenable sheets of material and, in particular, to bending glass sheets using a combination of shaped conveyor rolls, a lifting mold and a vacuum mold.
Shaped and tempered glass sheets are widely used as side windows or rear windows in vehicles such as automobiles or the like. To be suitable for such applications, flat glass sheets must be shaped to precisely defined curvatures dictated by the shape and outline of the frames defining the window openings into which the glass side or rear windows are installed. It is also important that the side or rear windows meet stringent optical requirements and that the windows be free of optical defects and reflective distortion that would tend to interfere with the clear viewing therethrough in their viewing area.
During fabrication, glass sheets intended for use as shaped windows in vehicles are subjected to thermal treatment to temper the glass for strengthening the same and increase the resistance of the shaped window to damage resulting from impact. In addition to increasing the resistance of the glass sheet to breakage, tempering also causes the glass sheet to fracture into relatively small, smooth surfaced fragments that are less injurious than the relative large, jagged fragments that result from the breakage of untempered glass.
The commercial production of shaped glass sheets for such purposes commonly includes heating flat sheets to the softening point of the glass, shaping the heated glass to a desired curvature and cooling the bent sheets in a controlled manner to a temperature below the annealing range of the glass. During such processing, a glass sheet is generally conveyed along a substantially horizontal path through a tunnel-type furnace, heated to its heat softening temperature and transferred into a shaping station adjacent the furnace, where the glass sheet is shaped. After shaping, the glass sheet is transferred to a cooling station where it is controllably cooled. The heat softened glass sheet may be shaped, for example by conveying the sheet over a series of transversely curved conveyor rolls as disclosed in U.S. Pat. No. 4,381,933 or by pressing the sheet between a pair of upper and lower shaping surfaces. In this later process, the upper surface is typically a full surface vacuum mold and the lower surface may be a full surface press as disclosed in U.S. Pat. No. 4,662,925, a segmented mold as disclosed in U.S. Pat. No. 4,272,274 or a ring type mold as disclosed in U.S. Pat. No. 4,830,650.
Within the past several years, considerable emphasis has been placed on the use of thinner glass sheets for automobile side windows as a means of reducing overall weight of the vehicle and obtain better fuel mileage. This has posed problems in shaping and tempering due to the lesser ability of thinner sheets, such as those having nominal thicknesses ranging from less than three millimeters to four millimeters, to retain heat and difficulties in utilizing the aforementioned glass sheet bending and tempering processes to process thin sheets. As the thickness of the glass decreases, the rate of heat loss increases and the heat initially imparted to such thin sheets is quickly dissipated upon leaving the heating atmosphere of the furnace and during the relatively cool shaping step. Attempts to solve these problems by initially overheating the thin glass sheets have not been entirely successful due to, among other things, the degradation of the surface quality of the finished glass as a result of roll ripple distortion and roll marking.
It would be beneficial to the glass sheet shaping art to develop a technique that combines the high speed operation of a roll forming method while maintaining the optical quality and shape control of glass obtainable by pressing a glass sheet against a vacuum mold.